Transport networks serve for the physical layer transport of high bitrate tributary signals. In particular, the signals transmitted over a transport network are encoded and multiplexed into a continuous bit stream structured into frames of the same length. Within this constant bitrate bit stream, the frames are repeated periodically with a frame repetition rate of typically 8 kHz and are structured according to a multiplexing hierarchy. An example of such a multiplexing hierarchy is SDH (Synchronous Digital Hierarchy, see ITU-T G.707 10/2000) where the frames are termed synchronous transport modules of size N (STM-N, where N=1, 4, 16, 64, or 256). The frames have a section overhead and contain at least one higher order multiplexing unit called virtual container VC-4. A VC-4 can either directly carry a tributary signal or a number of lower order multiplexing units like VC-12 or VC-3, which then carry tributary signals.
Virtual containers are transmitted from source to sink through an SDH network and therefore represent a “logical” path through the network. The sequence of identical VCs having the some relative position in subsequent frames forms a traffic stream along that path. Each VC contains a path overhead (POH) and a payload section referred to as container (C). The US equivalent of SDH is known as SONET (Synchronous Optical Network). Another well known transport network with similar multiplexing units is the recently defined Optical Transport Network OTN; see ITU-T G.709, 02/2001.
A very basic aspect in all types of transport networks is availability and reliability of service. In other words, a transport network must be very robust against any kind of failure and the average outage time must be very low. Hence, a transport network needs to provide the means and facilities to ensure sufficient availability. Typically, network mechanisms which ensure this availability are distinguished in protection and restoration. The common principle of both is to redirect traffic of a failed physical link or logical path over a spare resource.
Protection is a mechanisms where an already established protection path or link is assigned to one selected high-priority path or link (known as 1+1 or 1:1 protection, depending on whether there is low priority extra traffic on the protection resource or not) or a group of n such selected paths or links (known as 1:n protection). In the case of a failure, traffic can be re-routed very fast over the previously established protection resource under the sole control of the affected network elements in typically less than 50 ms. However; this requires a protocol between the affected nodes to signal and synchronize switch-over. Protection is a high-quality service restricted to few selected premium connections, which are typically charged at a higher price. Moreover, protection requires a high amount of spare resources compared with the amount of protected resources, i.e., 100% of spare capacity in the case of 1+1 protection.
Restoration refers to a mechanism, where the network searches for restoration capacity and establishes a restoration path only after service path failure. Rather than calculating the restoration path after failure, pre-calculated restoration routes can be used instead but with the actual cross-connection to establish the path performed after failure. Restoration mechanisms are more stringent in the usage of spare capacity and however, providing a masking of the failure at a lower speed, typically in the range of a few seconds as completely new paths through the network must be established.
In label switched packet networks, as opposed to transport networks, alternative label switched paths (LSPs) can already be implemented and then used in the case of a failure. The fundamental difference between transport networks and packet networks, where MPLS applies (Multi-Protocol Label Switching), is that in packet networks statistical multiplexing is used allowing over-subscription of links and that an LSP can be established without using any bandwidth. However, in transport networks, if a path is established, then by definition the full bandwidth requested by the path is consumed, independent of whether traffic is transmitted over this path or not. An LSP can be established before failure in MPLS, but not used until after failure, whereas this is not possible in transport networks.
The IETF proposal “RSVP-TE Extension for Shared-Mesh Restoration in Transport Networks” by G. Li et al, draft-li-shared-mesh-restoration-01.txt, addresses this issue and proposes an GMPLS extension where backup paths are pre-established in the management plane of the network but only activated after detection of a failure. This requires signaling messages to be exchanged in the management plane, which in a GMPLS controlled network is distributed across the entire network. This interaction of a decentralized network management is, however, a relatively slow process, which leads to restoration times in the range of at least some hundred milliseconds.